1. Field of the Invention
The present invention relates to a driving method for a matrix display type plasma display panel.
2. Description of Related Art
Nowadays, plasma display panel (referred to as PDP hereafter) having a plurality of discharge cells serving as pixels and arranged as a matrix is drawing attention as a two dimensional image display panel. The individual discharge cells are selectively discharged according to pixel data for the individual pixels based on a video signal, and a light emission caused by the discharge forms a display image on a screen in the PDP. In this operation, because each of the discharge cells use the discharge phenomenon to emit light, the discharge cells have only two states comprising a state for emitting light at the highest luminance, and a state for not emitting light. Namely, only luminance levels corresponding to two grayscales can be expressed. Thus, a grayscale drive based on a subfield method is conducted to provide an intermediate luminance display according to an input video signal on the PDP comprising the discharge cells.
The grayscale drive based on the subfield method a display period of each field is divided into N subfields, a light emission period (a number of light emissions) corresponding to a weight of each bit of the pixel data (N bits) is assigned to each subfield, and conducts a light emission drive for the PDP.
For example, when one field is divided into the six subfields SF1 to SF6 shown in FIG. 1, the following light emission periods are assigned to the subfields respectively.
SF1: 1
SF2: 2
SF3: 4
SF4: 8
SF5: 16
SF6: 32
Then, the light emission is conducted selectively in each of the subfields SF1 to SF6 according to the luminance levels represented by the input video signal. The intermediate luminance is visually sensed corresponding to the sum of the light emission periods through one field (SF1 to SF6). For example, when the discharge cell emits light only in SF6 of the subfields SF1 to SF6, the discharge cell emits light only for a period corresponding to “32” in one field, and an intermediate luminance corresponding to “32” is visually sensed. On the other hand, when the discharge cell emits light in the subfields SF1 to SF5 except for SF6, the discharge cell emits light in a period corresponding to “1”+“2”+“4”+“8”+“16”=“31”, and an intermediate luminance corresponding to “31” is visually sensed.
With these six subfields, there are 64 possible ways for combining the subfields to emit light, and the subfields not to emit light (light emission patterns). With these 64 ways of the light emission patterns, there are 64 ways of the sums of light emission periods through one field, and it is possible to express intermediate luminance corresponding to 64 grayscales.
As shown in FIG. 1, the relationships between light emission period and light-off period are inverted to each other for a discharge cell G31 corresponding to a pixel for representing luminance “32”, and a discharge cell G32 corresponding to a pixel for representing luminance “31” in a period of one field, for example. When one is viewing the screen of the PDP, if one sees the discharge cell G32 in a period from SF1 to SF5, and then moves the line of sight to the discharge cell G31, one continuously sees only the light-off periods of both of the them as shown as a broken line in FIG. 1. As a result, a dark line is visually sensed on a boundary between them as a false contour, and a problem of degrading the image quality occurs.
In view of the forgoing, it was devised to use only the seven light emission patterns shown in FIG. 2 out of the 64 light emission patterns described above for driving the PDP. With these light emission patterns, except for expressing a luminance of “0” as shown in FIG. 2, the discharge cell always emits light continuously from the first subfield SF1. Once the discharge cell switches to the light-off state, the discharge cell does not emit light in the following subfields thereafter through one field.
Thus, because the seven light emission patterns shown in FIG. 2 do not include light emission patterns whose relationships between light emission period and light-off period are inversed to each other for a period of the one filed, the false contour described above is not generated.
However, there is a problem that discharge should be caused even when the discharge cell is set to the light-off state as shown as black circles in FIG. 2, and the power consumption becomes large in driving the PDP.